1. Field of the Invention
This invention relates to fiber reinforced mirrors, and more specifically to the elimination of “print through” in fiber reinforced mirrors.
2. Description of the Related Art
Mirrors have been utilized in high precision optical systems for many decades. Critical to the successful application of these mirrors are material properties such as dimensional stability, low weight, high thermal conductivity, high stiffness, low coefficient of thermal expansion, etc. Different applications require various combinations of these properties. For example, many aerospace, telescope, airborne optics and fast-scanning optical applications require low weight, high stiffness and high dimensional stability. Beryllium is particularly attractive in this regard.
Beryllium mirrors are fabricated by consolidating beryllium powder by hot isostatic pressing (HIP) into a block, which is then machined into the desired shape of a mirror. Beryllium is a very expensive metal and has limited sources of availability. Furthermore, beryllium dust is toxic and has very limited near-net-shaping capability. A high precision beryllium mirror is therefore machined out of a beryllium block, thus wasting most of the beryllium by converting it into beryllium chips. The toxicity of the beryllium dust requires a special machine shop to meet rigid Occupational Safety and Health Administration (OSHA) requirements for safety, which adds to its expense. A material that is nontoxic and cheaper than beryllium metal is desired.
As part of a concerted effort to eliminate the use of beryllium materials, fiber reinforced matrix composite mirrors have been under development for over twenty years. As shown in FIG. 1a, a matrix material 10 such as carbon (U.S. Pat. Nos. 4,451,119 and 4,915,494), polymer (U.S. Pat. Nos. 4,842,398, 5,178,709, 5,907,430 and 6,431,715 ceramic (U.S. Pat. Nos. 4,256,378 and 5,382,309) or metal is reinforced with a graphite fiber weave (or Silicon Carbide, Boron, etc) 12 to form a lightweight, thermally conductive and stiff substrate 14. To provide the necessary stiffness, several hundred to many thousand fibers 16 are bundled into “tows” 18, which are woven into a desired pattern. A fiber has a diameter of at least one micron and more typically ten to twenty microns and a tow has a diameter of 0.5 to 7 mm. The weave pattern has a relatively coarse structure or texture as defined by the center-to-center spacing S1 of alternate tows, suitably 1.5 mm to 17 mm, or the average diameter D1 of the inscribed circle between the centers of in-phase tows to provide the requisite stiffness. In the case of a weave, S1 and D1 are equal.
A thin layer 20 of the un-reinforced matrix material is formed on the substrate 14 and processed to create an optical quality surface 21. A reflective optical coating 22 (gold, silver, aluminum. etc) is evaporated onto the optical surface to define a mirror surface that conforms in shape to the optical quality surface. The un-reinforced layer does not contribute appreciably to the strength or stiffness of the composite mirror. Hence, to minimize weight the layer is only thick enough, typically about 0.1 mm, to define the optical surface.
The fiber reinforced matrix provides low weight and high stiffness approaching that of beryllium without high cost or toxicity. However, unless the un-reinforced layer is made very thick and thus very heavy, the composite mirror will, over time and temperature cycling, produce high spatial frequency deformations 24 in the optical surface 21, hence reflective optical coating 22 as best shown in FIG. 1b. The coefficient of thermal expansion (CTE) and stress mismatches within the reinforced substrate, and between the substrate and un-reinforced layer, creates a stress/strain pattern that emulates the coarse texture of the weave pattern. The un-reinforced layer transfers the stress/strain pattern to the optical surface. This is known as “print through” and effectively degrades the optical properties of the mirror. As a result, fiber reinforced mirrors have not achieved successful commercialization to replace beryllium and other isotropic metal mirrors.
There remains an acute and present need to solve the print-through problem for fiber reinforced matrix composite mirrors without degrading weight, stiffness or dimensional stability.